A vertical cavity surface emitting laser (VCSEL) is a laser device in which an optical resonator is formed in a direction vertical to a substrate surface and a laser beam is output in the direction vertical to the substrate surface. Usually, a current constriction structure is formed in the VCSEL to concentrate a current to a light emitting region.
In many cases, a cavity structure formed by oxidizing the outer peripheral side of an AlAs (aluminum arsenic) layer is utilized as the current constriction structure. More specifically, the AlAs layer is oxidized by holding, in water vapor heated to 400 to 500° C., a substrate on which a multilayer body (oxidation object) processed into a mesa-like shape such that the AlAs layer is exposed at a lateral surface of the multilayer body. In that case, a temperature distribution inside the substrate is desirably within ±1° C.
A method disclosed in Japanese Unexamined Patent Application Publication No. 2011-18876 is known as a process for making the temperature distribution inside the substrate more uniform. According to the method disclosed in Japanese Unexamined Patent Application Publication No. 2011-18876, an upper surface of a radiation plate on which the substrate is placed is processed into a spherical shape corresponding to a warp of the substrate on which the oxidation object is formed, the warp being caused during thermal oxidation.